The field of the present invention is in the radar system simulation art and more particularly that of radar system simulations employing electronic countermeasure jamming techniques.
The use of electronic countermeasure (ECM) systems for jamming radar systems is well known. The effectiveness of the ECM equipment depends, to a large extent, upon a knowledge of the operational characteristics of the radar that the jammer is designed to defeat. In order to evaluate the effectiveness of the ECM, current state-of-the-art methods employ a real-time computer controlled radar simulator. Some such simulators utilize an anechoic chamber while others can function effectively without a chamber. See, for example, U.S. Pat. No. 3,982,244. Occasionally, an ECM technique is devised that requires such precise amplitude and phase control of the microwave signals that these new tolerance demands exceed the relatively coarse accuracy capabilities of the simulation systems. An example of such is the Cross Eye ECM Technique.
The Cross Eye jamming technique works on the interferometer principle. In its simplest, conceptual definition, the technique requires radiation from one source through two physically separated antennas on an aircraft (usually at the ends of the wings). If the amplitudes of the radiated signals are nearly equal, deep interferometer nulls will be formed. If phase is precisely controlled, the position of the nulls can be positioned wherever desired. The intent of the Cross Eye technique is to position a null on half of a radar antenna and provide a residual of energy on the other half (see FIG. 1). This, in turn, creates a tracking error within the radar. Should the radar use phase comparison to develop tracking information, the technique remains viable because the radiated phase front in the region of the interferometer nulls shifts radically.
Previously, the evaluation of this technique was accomplished through flight testing. However, implementation of flight testing has been found to be expensive and extremely difficult because (a) deep interferometer nulls must be formed and (b) the null positioning must be precise, thereby requiring flight profiles that are difficult to maintain. When the two signal sources have a 180.degree. phase difference, the deep nulls will result in the jamming energy nearly cancelling each other on the side of the radar receive antenna that generates a tracking error. This, in turn, means that high jammer powers are required. In addition, the radiated signals must be very close in amplitude. The precise null positioning requires precise phase control.
The specific point of a 180.degree. relative phase difference of the two signals represents a mathematical singularity. Under such conditions, the null is directly on the center of an antenna and no tracking error is generated. The radiated signals effectively cancel to the extent that the amplitudes are the same. This singularity is important because it rules out the use of a single ECM antenna with cancellation internal to the jammer. Laboratory simulation in an anechoic chamber cannot be accomplished for two reasons. The first is that the scaling of distance places the ECM antennas on top of each other if realism is to be retained. The second is that the equipment in the chamber must transmit and receive at the same time. To accomplish this, two physically separated antennas are required to obtain adequate isolation. Unfortunately, the ECM hardware places its jamming null on the transmit antenna and not on the desired receive antenna.
Any simulation of the Cross Eye or similar technique would have to consider the conditions likely to cause problems, such as glint, thermal expansion, and devices that cause phase shifts. Mathematical formulas for basic Cross Eye technique requirements can be extracted from discussions of radar glint. These glint equations lead to simulator requirements in the neighborhood of .+-.0.1.degree. relative phase accuracy and .+-.0.1 db relative amplitude accuracy to be simulated at actual radar frequencies. Depending upon an aircraft's location relative to the radar, a phase accuracy of .+-.0.1 db relative amplitude accuracy might be considered marginal. For design purposes, however, a .+-.0.1.degree. phase accuracy and a .+-.0.1 db amplitude accuracy are sufficient to identify the simulation constraints.
Thermal expansion and vibration do cause serious simulation constraints. The phase shift .phi. caused by a differential length l is: EQU .phi.=360.degree. (fl/c)
where c is the velocity of light and f is the frequency. The differential length of a 0.1.degree. phase shift at 10 Ghz is found to be 8.3 microns. Unless phase is recalibrated at rapid intervals, thermal expansion will prevent simulation. If recalibration occurs only once every 100 seconds, the thermal expansion rate must be kept below 0.083 microns/second. If recalibration occurs once every millisecond (i.e., nominally once per radar pulse), then the allowable rate is 8.3 millimeters/second. Vibration causes similar problems although the mechanism is the addition of effective capacitances and/or inductances in the microwave circuits. Care must be taken to limit the amount of vibration and rate of vibration between recalibration of phase.
Devices that influence signal levels, such as PIN diode modulators, also cause phase shifts. This occurs because of stray capacitances and inductances. Such effects are greatly magnified due to the microwave frequencies involved.
Active components such as Traveling Wave Tubes (TWTs) can cause phase shifts as a result of minor power supply variation. In addition, devices other than passive linear components generally introduce a relatively large phase shift whenever operated over a wide dynamic range.
From the above discussion, it is apparent that any simulation of the Cross Eye or similar technique would require (a) rapid phase and amplitude recalibration, (b) minimal use of active components, (c) immediate down conversion to an intermediate frequency in order to provide adequate components (i.e., the effects of stray capacitances and inductances can be reduced significantly), and (d) the restriction of the dynamic range applied to critical components.
It is also necessary to synthesize portions of the antenna pattern and to assure that signal levels during recalibration are far above thermal noise levels to permit accurate relative phase and amplitude measurements. Only after the antenna pattern is synthesized is it reasonable to introduce the path losses of the radar range equation.